Which is best at mitigating browser fingerprinting? Firefox (with or without arkenfox)? Librewolf? Mullvad browser?

Technically, the best way to blend in is to avoid changing the behaviour much from the default. I would still advise the below settings because they do improve your security, and anti-fingerprinting against naive first-party fingerprinting scripts (all 3rd party scripts/iframes should be blocked, see below: uBlock Medium/Hard). If you need protection against advanced fingerprinting use Tor/Mullvad browser.

uBlock:

• Change uBlock blocking mode to Medium or Hard using the instructions on their Github wiki. Can cause site breakage on shitty websites (eg sites that import large JS libraries from remote sources). It is a substantial improvement over default, see the wiki for medium mode: https://github.com/gorhill/uBlock/wiki/Blocking-mode:-medium-mode

• Enable filterlist Privacy>Block Outside Intrusion to LAN (Access to LAN is used to fingerprint or by threat actors during reconnaissance phase of hacking)

• Consider enabling other filterlists included in uBlock. Try to minimize enabling extra lists from the default to avoid further fingerprinting.

Librewolf:

• Enable limiting of referrers under LibreWolf Preferences>Privacy>Limit cross-origin referers

• Enable letterboxing under LibreWolf Preferences>Fingerprinting>Enable letterboxing

Neptr covered it better than I could've. I also added privacy badger though I'm not sure that does anything🤷